Attribute macros can't be applied to statements

[abonander]

If my_attr is an attribute proc macro which otherwise resolves in a given context,
#[my_attr] let foo: i32; will fail to resolve.

I'm on mobile because my home internet is down so I can't really provide a better example. Will update later if necessary.

cc @jseyfried

[Mark-Simulacrum]

@abonander Could you provide a full example, preferably on playpen, but since this involves proc macros I suspect that might not be possible?